(a) Field of the Invention
The present invention generally relates to a pipe and in particular to a galvanic corrosion resistant insulating pipe in which galvanic corrosion and crevice corrosion, which otherwise occurs when pipes made of different metals are joined to each other for piping oil country tubular goods, is prevented from occurring and adhesion of an insulating ceramic film on a base material of the pipe is enhanced.
(b) Related Art
Various metal pipes are used in oil wells, including tubing for conveying crude oil and natural gas from the underground production formation up to the ground; casing which is provided around the tubing for protecting bored wells; steam injection pips which increase the pressure in the oil formation; and CO.sub.2 pipes for secondarily enhanced oil recovering fossil oil. In oil wells, these metal pipes are disposed in the ground so that they are perpendicular or at an angle substantially perpendicular with respect to the surface of the ground and extend downwardly into the underground at several thousand meters for exploiting or producing crude oil and natural gas. These pipes used herein will hereinafter referred to as oil country pipes.
In general, corrosiveness at a deep level in oil wells is high since the temperature is high. Conversely, corrosiveness in a shallow level in the well is low since the temperature is low. Allowing for economy, high corrosion resistant metal pipes, which are made of stainless steels, Ni based alloys, Ti and Ti based alloys, are often used in the deep level in the well and low corrosion resistant pipes, which are made of carbon steels and, low alloy metals, are often used in the shallow level in the well.
If the low corrosion resistant metal pipe is simply joined to the high corrosion resistant pipe, a potential difference will occur by the contact between different metals, resulting in galvanic corrosion. Corrosion of the low corrosion resistant metal pipe due to galvanic corrosion proceeds at a rate which is 2 to 10 times as higher than the corrosion rate of a single usual carbon steel pipe.
On the other hand, hydrogen is generated on the side of the high corrosion resistant metal pipe. The resultant hydrogen will enter into the pipe to induce hydrogen brittlement. Further, crevice corrosion will occur in the crevices of a joint so that corrosion is promoted by galvanic corrosion.
In order to overcome these problems, the high and low corrosion resistant metal pipes are not directly joined to each other, and a metal pipe made of duplex stainless steel having a corrosion resistance which is intermediate between those of the high and low corrosion resistant metal pipes is interposed between the high and low corrosion-resistant pipes for preventing the corrosion from occurring.
However, it is not possible to completely prevent galvanic corrosion between metal pipes and crevice corrosion from occurring even if an intermediate metal pipe is interposed therebetween. Only the corrosion rate is made slightly lower.
It is practically impossible to completely seal the joined portions when pipes are joined to each other by a blot joining structure. Open ended crevices are inevitably formed. Since there is almost no flow of liquid in the crevices which are formed in the bolt jointed structure, hydrated ions, which are generated by the corrosion of a metal such as iron, have tendency to stay at a high concentration. The pH is remarkably lowered and more severe corrosion than the base material may occur. In other words, crevice corrosion will occur in an environment which is milder than the corrosion environment of the base material. Occurrence of crevice corrosion will lead to stress corrosion cracking.
In order to solve the above mentioned problem, Japanese Unexamined Patent Publication No. Hei 1-199088 discloses that a bolt joined portion of oil country pipes of material with thread coupling containing 7.5% by weight or more of Cr is coated with a non-metallic layer having a thickness of 1 to 100 .mu.m to protect the joined portion from the corrosion environment for preventing crevice corrosion from occurring. However, coating of the non-metallic layer in accordance with the art taught by the above mentioned patent publication provides an advantage as follows:
Non-metallic layers are electrically conductive or insulating material. Even if the non-metallic layer is an electrically conductive material, formation of non-defective film having no pinholes is practically impossible. Since a number of pinholes are usually formed in the film, corrosion of the defective portions of the film can not be prevented from being promoted. Even if the non-metallic layer is the insulating material, crevice corrosion may occur depending upon the coverage ratio of the film. In either case, the art disclosed in the above mentioned patent publication can not sufficiently prevent crevice corrosion.
Although in order to reduce such defective portions of the film, it suffices to reduce the number of pinholes by thickening the film, the increase in film thickness causes lowering of adhesion of the film to a base material and the film may be separated from the base material by shear stress due to the difference between the Young's moduli of the base material and the coating film, which is caused on fastening the joint.